The subject matter disclosed herein relates to motion control systems and, more specifically, to a method and apparatus for determining coil current references for drive coils used to control operation of multiple independent movers traveling along a track in a linear drive system.
Motion control systems utilizing movers and linear motors can be used in a wide variety of processes (e.g. packaging, manufacturing, and machining) and can provide an advantage over conventional conveyor belt systems with enhanced flexibility, extremely high speed movement, and mechanical simplicity. The motion control system includes a set of independently controlled “movers” each supported on a track for motion along the track. The track is made up of a number of track segments that, in turn, hold individually controllable electric coils. Successive activation of the coils establishes a moving electromagnetic field that interacts with the movers and causes the mover to travel along the track. Sensors may be spaced at fixed positions along the track and/or on the movers to provide information about the position and speed of the movers.
Each of the movers may be independently moved and positioned along the track in response to the moving electromagnetic field generated by the coils. In a typical system, the track forms a closed path over which each mover repeatedly travels. At certain positions along the track other actuators may interact with each mover. For example, the mover may be stopped at a loading station at which a first actuator places a product on the mover. The mover may then be moved along a process segment of the track where various other actuators may fill, machine, position, or otherwise interact with the product on the mover. The mover may be programmed to stop at various locations or to move at a controlled speed past each of the other actuators. After the various processes are performed, the mover may pass or stop at an unloading station at which the product is removed from the mover. The mover then completes a cycle along the closed path by returning to the loading station to receive another unit of the product.
Typically, each mover includes one or more permanent magnets mounted to the mover which, in combination with the drive coils spaced along the track, form a linear drive system. A motor controller generates a voltage having a variable amplitude and variable frequency which, in turn, results in a desired current flowing through each drive coil. The current flowing through the drive coil generates an electromagnetic field which interacts with the magnetic field produced by the permanent magnets to cause the movers to travel along the track.
As each mover travels past a coil, the permanent magnets create a back-emf voltage in each coil that is counter to the applied voltage. The back-emf voltage interacts with the applied voltage and current in each coil. If the back-emf voltage and the current in the coil are both sinusoidal waveforms, the interaction between the back-emf and the current in the coil is smooth, meaning there are no force pulsations. If, however, one or both of the waveforms are non-sinusoidal, then an undesirable force pulsation may be present on the mover. In addition, the permanent magnets mounted on the mover attempt to align themselves with the maximum amount of ferromagentic material present on the track, creating a cogging force. The force pulsations due to non-sinusoidal waveforms combines with the cogging force to generate an undesirable force on the mover as it travels in the linear drive system.
Several factors impact the shape of the waveforms in the linear drive system. Some of the factors include the design, shape, and placement of the magnets on the mover as well as the design, pitch, and placement of the coils along the track. These factors make it difficult to design an ideal linear drive system with purely sinusoidal waveforms and no cogging force. Still additional factors include the size of the mover and the number of coils with which the mover will interact at one time. These factors impact not only the force pulsations on the mover but also whether the currents are balanced between coils and the amount of copper losses in the coils.
A designer for the linear drive system must balance the competing effects of the different factors when designing the linear drive system. The designer must further balance manufacturing and material costs associated with the various design factors. As a result, a linear drive system typically has current and back-emf waveforms that are not purely sinusoidal as well as some amount of togging force between the magnets in the movers and the laminations of the track. In addition, there may be variations in the linear drive system due, for example, to variations in placement of coils on the track along a straight segment and along a curved segment. Therefore, movers of different sizes and of different construction will interact differently with the coils along a track and may further interact differently along different sections of a single track.
Thus, it would be desirable to provide an improved method and system for providing current to the coils in a linear drive system. It is also desirable to provide different methods of regulating the current to the coils at different segments of the track according to the application requirements.